1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copying machine, a facsimile machine, etc., configured to form an image on a recording medium.
2. Description of the Related Art
In recent years, there has been a strong demand for a reduction in power consumption of an image forming apparatus such as a printer, a copying machine, a facsimile machine, etc. In particular, there is a demand for a reduction in power consumption in a waiting state in which no operation is performed. To meet the demand for a reduction in power consumption, many image forming apparatuses have a power-saving operation mode in which electric power is supplied only to units that really need electric power in the waiting state. In a known example of a power-saving operation mode, electric power is supplied only to a control unit including a CPU and supplying of electric power to the other units is stopped.
An example of a power-saving operation mode is described below with reference to FIG. 1. FIG. 1 is a control block diagram illustrating how various control units in the image forming apparatus are connected to each other. As shown in this control block diagram, the image forming apparatus includes an engine control unit 104, a controller 103, and a scanner control unit 102. In accordance with information received from the controller 103, the engine control unit 104 controls a whole image forming unit (hereafter also referred to as the engine unit) in terms of various operations including an operation of feeding, conveying, and ejecting a recording medium, an operation of forming an image on the recording medium, an operation of fixing the image formed on the recording medium, etc. The engine control unit 104 is connected to a power switch 403 used to turn a power supply on or off, a door switch 401 configured to detect opening/closing of a door, and a sensor 402 configured to detect a status of the apparatus.
The controller 103 configured to receive information from a host computer 105 connected to the image forming apparatus, and, in accordance with the received information, control the operation of the engine control unit 104 and the scanner control unit 102 via communication with them. The controller 103 notifies the host computer 105 of the operation status of the image forming unit and the scanner unit 201. In accordance with the information from the controller 103, the scanner control unit 102 controls the operation of the scanner unit 201. The scanner unit 201 connected to the controller 103 functions as a reading apparatus for inputting image information. The scanner unit 201 includes an image sensor configured by combining an optical system to focus light to form an image of a document and a CCD line sensor such that the document image can be read while scanning the CCD line sensor. An automatic document feeder (ADF) 203 serves to automatically feed a document to the scanner unit 201. An operation unit 202 is an operation panel including an operation button and an LCD (or LED). A user is allowed to input data or a command associated with an operation of the scanner unit 201 or the ADF 203 by operating an operation button on the operation panel.
The scanner control unit 102 includes a microprocessor unit (MPU) 102a serving as a control processing unit, the controller 103 includes an MPU 103a serving as a control processing unit, and the engine control unit 104 includes an MPU 104a serving to control the engine. Hereinafter, the MPU 103a in the controller 103 will also be referred to as the controller MPU 103a, and the MPU 104a in the engine control unit 104 will also be referred to as the engine control MPU 104a. The controller MPU 103a transmits and receives information to and from the MPU 102a in the scanner control unit 102 and the engine control MPU 104a by performing bidirectional serial communication.
FIG. 2 illustrates a circuit including the MPUs shown in FIG. 1 and associated circuit elements. If the engine control MPU 104a turns on a switching element 501, electric power is supplied to the scanner control unit 102 (MPU 102a) and the controller 103 (MPU 103a). In a power-saving operation mode, the MPU 104a turns off the switching element 501 to deactivate the scanner control unit 102 (MPU 102a) and the controller 103 (MPU 103a) thereby reducing power consumption. Note that, for example, an FET may be used as the switching element 501.
FIG. 3 illustrates another example of a configuration. The configuration shown in FIG. 3 is different from that shown in FIG. 1 in that the power switch 403 used to turn on/off the power supply, the door switch 401 configured to detect opening/closing of a door, and the sensor 402 configured to detect an access of a user are connected to the controller 103. FIG. 4 illustrates a circuit including the MPUs shown in FIG. 3 and associated circuit elements. In the power-saving operation mode, the controller MPU 103a turns off the switching element 501 thereby cutting off the electric power to the engine control unit 104 (MPU 104a).
Instead of the above-described configuration in which the electric power to the control units is stopped in the power-saving operation mode, the oscillating operation of the MPU may be stopped while maintaining the supplying of electric power to the control unit (control MPU) as disclosed, for example, in Japanese Patent Laid-Open No. 2000-307784.
In the circuit configuration shown in FIG. 2, a reduction in power consumption can be achieved by cutting off the electric power to the scanner control unit 102 and the controller 103 in the power-saving operation mode. However, because the MPUs do not operate in the power-saving operation mode in which no electric power is supplied to the MPUs, it is impossible to control the operation such that in response to a trigger from the host computer 105 or the operation unit 202, the operation mode is returned from the power-saving operation mode into a normal operation mode in which the electric power is turned on for all control units. In the configuration shown in FIG. 2, to switch into the normal operation mode from the power-saving operation mode, it is necessary to turn on either one of the power switch 403, the door switch 401, and the sensor 402. Therefore, in the configuration shown in FIG. 2, a troublesome process is necessary to return into the normal operation mode, which leads to a reduction in usability.
On the other hand, in the circuit configuration shown in FIG. 4, in the power-saving operation mode, the controller MPU 103a controls supplying of electric power such that supplying of electric power is stopped only to the engine control unit 104. This makes it possible to return into the normal operation mode from the power-saving operation mode in response to turning-on of any one of the host computer 105, the operation unit 202, the power switch 403, the door switch 401, and the sensor 402. Thus, compared with the configuration shown in FIG. 2, a less troublesome process is required to return into the normal operation mode and high usability is achieved. However, in the configuration shown in FIG. 4, electric power is supplied to the controller 103 and the scanner control unit 102 in the power-saving operation mode, and thus the amount of reduction in electric power in the power-saving operation mode is not sufficient compared to that achieved in the configuration shown in FIG. 2.
That is, in the above-described control scheme in which electric power is always supplied to a predetermined particular control unit (MPU), and this MPU controls switching of the operation mode (between the normal operation mode and the power-saving operation mode), it is difficult to achieve simultaneously both high usability and low power consumption in the power-saving operation mode.
In the scheme disclosed in Japanese Patent Laid-Open No. 2000-307784 in which the oscillation of the MPU serving to control the engine is stopped in the power-saving operation mode, it is difficult to achieve a sufficient reduction in power consumption in the power-saving operation mode, and it is difficult to achieve simultaneously both high usability and low power consumption in the power-saving operation mode. Furthermore, in the scheme which the oscillation of the MPU serving to control the engine is stopped in the power-saving operation mode as in the scheme disclosed in Japanese Patent Laid-Open No. 2000-307784, when the MPU serving to control the engine is in the state in which the oscillation is stopped, there is a possibility that an erroneous or runaway operation of the MPU is not detected. If such an abnormality occurs in the MPU in the state in which the oscillating operation is stopped, the abnormality can cause electric power to be incorrectly supplied to a device controlled by the engine control unit, which may lead to a failure of the apparatus.